This invention relates in general to a method and apparatus for coating hollow articles on their interiors and in particular to a new and useful method for varnishing the interior of metallic tubes and cans.
The invention relates particularly to a device for varnishing hollow bodies from inside, especially metallic tubes or cans. The tubes or cans are supplied continuously by means of a delivery system through a handing over station of a revolving drum. They are guided past a unit which varnishes inside spaces and it includes a spraying nozzle device, upon which a number of spraying nozzles are disposed. The nozzles extend into the hollow bodies and they can be slid along so as to be guided afterwards into a guiding-away system through a receiving head end.
Such devices for varnishing hollow bodies from inside are known and are delivered for example by the firm Sprimag GmbH, D-7312 Kirchheim-Teck under designations such as HIL-26, HIL-28. The hollow bodies which are twisted on the drum, receive the varnish layer by the fact that the spraying nozzles submerge into the hollow bodies. The spray nozzle support, on which the spray nozzles are attached, follows the turning of the drum supporting the hollow bodies. This occurs during a working phase, when the spraying nozzles are submerged into the hollow bodies. During the extraction of the spraying nozzles out of the hollow bodies the varnish is sprayed onto the inner surface of the hollow bodies.
The quantity of varnish flowing through is adjusted as a function of the twisting speed of the spraying nozzles and thus an even layer of varnish showing the desired thickness can be obtained. A duplex procedure is used for varnishing hollow bodies from inside. That means that always two spraying nozzles carry out one identical working process, and thus always two hollow bodies are varnished together. Thus for varnishes to be sprayed on in two layers, four spraying nozzles are necessary. It is possible as well to lead the hollow bodies, which have received a varnish layer, away from the drum by means of a take-off station after the two spraying nozzles pass through. They are then directed through a drying furnace and then back onto the drum for a second varnish layer applied by the third or fourth spraying nozzle.
In the case of such installations for varnishing hollow bodies from inside, it is inevitable, that the spraying causes a fog of varnish. This fog of varnish is sucked off by suction devices disposed outside of the area through which the hollow bodies are moved.
This avoids an exceeding pollution of the machine parts next to the varnishing device by a fog of varnish which would settle down on them. However, the spraying nozzles, which move continuously in the fog of varnish nevertheless are polluted in a very short time, so that the good quality of the varnish layer is not guaranteed, because the pollution can hinder the varnish from leaving the spraying nozzle. In order not to incur the risk of losing quality, the spraying nozzles have to be cleaned regularly with a certain frequency. For this end the installation is switched off, the spraying nozzles are brought into the position in which they would extend from the hollow bodies and then each individual spraying nozzle has to be cleaned by hand. This requires a workman who frees the spraying nozzles, which is very difficult because they are washable. This results in the installation being out of service for a very long time.